fix(think,agents): keep a restart-interrupted regeneration on its own branch

[threepointone]

Follow-up to #2413 (#2028). That PR noted one gap: a regeneration interrupted after partial output takes the "continue" recovery path. We had described that as continuing on the right branch instead of restarting. Writing a test for it showed a real bug instead.

Problem

When a restart (Durable Object eviction or a deploy) interrupts a regeneration that has already streamed part of its answer:

1. Orphan recovery rebuilds the partial answer from the stored stream chunks. It appends it with no parent (persistReconstructedOrphan then appendMessage(message)), so it lands under the latest leaf, which is the response being replaced. The tree becomes user, old answer, new partial instead of the new partial sitting beside the old answer.
2. The continuation extends that leaf, so the model prompt is user, assistant: <old answer>, assistant: <new partial>, followed by "continue". The replaced answer is back in the context, which is exactly what Think regeneration includes the previous response in the model prompt #2028 fixed for live turns.

The live interruption path (stall or transient error) already passes branchParentId into persistPartial (#2413). Only the restart paths went wrong: fiber recovery, and a client's resume ACK after hibernation. Neither has the turn's fiber snapshot where the orphan is persisted.

Fix

• agents: ResumableStream.start() accepts an optional parentMessageId, stored in the stream row's chat metadata next to messageId and originMessageIds. getStreamParentMessageId(streamId) reads it back.
• think: _streamResult records the turn's parentId (set for regenerations) when it opens the stream. _persistOrphanedStream appends the reconstructed message under that parent. Both orphan paths go through it. On an agents release without the getter it falls back to the current behavior.

Kept the continue behavior rather than restarting the answer: the user has already watched the partial answer stream, and every other interrupted turn keeps and continues its partial.

Tests

• continues a regeneration interrupted after partial output on its new branch (#2028): a restored streaming row with a recorded parent. It asserts the partial lands beside a-previous, the old answer is untouched, and the continuation prompt is user then assistant: New partial, without the old answer. Mutation check: disabling the parent lookup fails it.
• keeps a recovered partial with no recorded parent on the latest leaf: turns that do not branch are unchanged.
• The existing regeneration test now also asserts that a live regeneration records its user message as the stream's parent, and a normal turn records none.
• agents: the parent survives restore(), and it is null for a non-branching stream or a missing row.
• Think's full Workers suite passes (1,222 tests), as does pnpm run check.

---

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🦋 Changeset detected

Latest commit: e382893

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@cloudflare/think	Patch
agents	Patch
@cloudflare/agent-think	Patch

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[agent-think]

✅ agents import sizes: no significant changes (c2a7bc54 → e3828933, workflow run)

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

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